SEISMOSTRATIGRAPHY AND SEDIMENTOLOGY OF HOLOCENE SEDIMENTS OFF GRANDE-RIVIERE-DE-LA-BALEINE, SOUTHEASTERN HUDSON-BAY, QUEBEC

The regional distribution of Holocene sediments of eastern Hudson Bay off the Grande Riviere de la Baleine mouth was mapped using a grid of reflection seismic lines (approximately 300 km long and covering an ar ea of approximately 800 km2) and data from 7 piston cores. Based on th e seismic records and piston cores, 4 stratigraphic units overlying th e Proterozoic bedrock (unit 1) were defined and interpreted: (unit 2) glacial till deposited by a westward flowing ice sheet, (unit 3) rhyth mically bedded clays and silts presumably deposited in glacial Lake Oj ibway, (unit 4) postglacial marine muds deposited in the Tyrrell Sea o verlain by undifferentiated modern marine muds, and (unit 5) distal fl uvio-deltaic sediments from Grande Riviere de la Baleine. Similar stra tigraphic units have been described onshore. Textural and geochemical analyses suggest that unit 3 rhythmites are true varves; dark ''summer '' laminae were deposited mainly by underflows during the open water s eason, and light ''winter'' laminae were deposited by overflows-interf lows along thermal stratifications under a seasonal ice cover. Unit 5 covers approximately 400 km2 and occurs as a deltaic constructional we dge protruding as far as 11 km offshore of the Grande Riviere de la Ba leine entrance with thicknesses reaching 30 m along the coast. It was deposited between 3500 BP and the present from remobilization of glaci al sediments farther upstream due to river downcutting during emergenc e.